Mixmaster has long used 1024-bit RSA with a packet format that allows
a maximum of 20 hops; each encrypted with a different RSA key.  The
data for each hop occupies 512 bytes.

Given the declining protection offered by a key size from the 1990s I
decided to investigate adapting mixmaster to use larger RSA keys
at a cost of reducing the longest chain to 10 hops.

The new code can use key sizes of 2048, 3072 and 4096 for RSA.
E.g. 10 hops of 4096; or 2 of 1024 and 9 of 4096.
The default size is 4096 bits.

When using the larger RSA keys (2048 and up) the symmetric crypto of
3DES CBC is augmented by adding AES-256 on top of it.  The change in version
3.0.3 from the 3.0.2 series is that CTR chaining mode is preferred over CFB.
Beside cryptographic advantages this makes it easier to develop compatible
software that doesn't have to use OpenSSL.

Code location:
http://www.zen19351.zen.co.uk/mixmaster303/

Further reading:
http://www.freehaven.net/anonbib/cache/mixmaster-spec.txt
https://crypto.is/blog/packet_formats_1
https://crypto.is/blog/tagging_attack_on_mixmaster
http://www.nsa.gov/business/programs/elliptic_curve.shtml